Bascule bridges need to possess the ability for the bridge operator to quickly and reliably change span orientation to alternately permit the passage of land and waterway traffic. Bascule bridges must be able to open and close on demand; yet, at the same time they should be as rigid in their closed position as a fixed span. The system intended to secure bridge leafs in the closed position is customarily known as a “Static Stabilizing System” and it normally includes components such as span locks, live load shoes, anchorages, machinery brakes and, in some instances, tail locks. Live load shoes and anchorages are the heart of the system because they transfer the traffic loads directly from the leaf to the pier. Both of these components are frequently subjected to shock loads both when the leaf is closing into its fully seated position and when the leaf is closed with heavy traffic crossing the span. Leaf pounding, or bounce, resulting from vehicle passage and from the leaf slamming down hard onto its seats with each closing, imparts shock loads to the movable leaf structure as well as to the pier and supporting structures. Repetitive shock loading causes abnormal wear of the live load shoes, anchorages and their respective seats.
Over years of bridge service, the excessive wear, coupled with normal thermal expansion and contraction, corrosion and deterioration, diminishes the ability of the components to act in concert with one another and function as a system. Many serious problems result, including distress and failures in machinery components, which are directly attributable to poorly adjusted and maintained static stabilizing components. For example, there is spalling and cracking of live load seat concrete support columns from frequent high shock loads due to slamming the leaf onto its seat during closing plus repetitive shock loads from vehicles passing across the span; fracture of pinion or rack teeth in the operating machinery caused by cyclic shock loads on both faces of a tooth while the leaf is closed; and, fracture of trunnion bearing bushings caused by extremely high loads and repetitive shocks due to poorly adjusted live load shoes, anchorages and span locks.
Frequent periodic bridge maintenance is required to assure that live load shoes are in firm contact with their seats and the anchorages are adjusted as intended. Adjustment usually requires complete removal of each live load shoe and anchorage in order to insert shims of proper thickness between them and their respective supporting structures. When the leaf is closed and the span locks are engaged, the live load shoes should not have any clearance with their seats. Correct anchorage adjustments depend on the particular design of the bascule leaf. Some do not require any clearance with their seats, and others require a slight, e.g. 0.020 to 0.050 inch clearance. Live load shoe and anchorage adjustment is tedious and difficult because the components are cumbersome, weighing hundreds of pounds, and because anchorages are often situated in inaccessible locations with respect to the bascule leaf. Adjustment is a time-consuming, labor-intensive process that requires the span be closed to all vehicular and waterway traffic, and this results in inconveniences to the traveling public. Failure to keep the static stabilizing system properly adjusted is an invitation to more serious trouble and damage, greater repair and replacement costs, and lengthier periods of inconvenience to users of the bascule bridge.